1. Field of the Invention
The present invention relates to an electrophotographic photoconductor, and more particularly to an electrophotographic drum-shaped photoconductor with improved mechanical durability. The present invention also relates to an image forming method and apparatus using the above-mentioned electrophotographic drum-shaped photoconductor. The present invention is applicable to a copying machine, facsimile machine, laser printer, and direct digital printing master making apparatus.
2. Discussion of Background
The electrophotographic process using an electrophotographic photoconductor, which is utilized in the copying machine, facsimile machine, laser printer, and direct digital printing master making apparatus, includes at least the steps of uniformly charging the surface of the photoconductor, exposing the charged surface of the photoconductor to light images to form latent electrostatic images thereon, developing the latent electrostatic images with a developer to visible images, transferring the visible images to a transfer sheet, fixing the images to the transfer sheet, and cleaning the surface of the photoconductor.
In line with the trend toward the personal use of copying machine, facsimile machine, and laser printer, there is an increasing demand for improvement of durability of such electrophotographic apparatus so as to be free of maintenance, and improvement of stability of the obtained image quality.
Organic photoconductors are now widely used in the electrophotographic process because of various advantages over other photoconductors, for example, because of low manufacturing cost, high degree of freedom in the designing of the photoconductor, and no pollution problems.
There are conventionally known an organic photoconductor comprising a photoconductive resin such as polyvinylcarbazole (PVK); an organic photoconductor comprising a charge transport complex material such as polyvinyl carbazole (PVK)-2,4,7-trinitrofluorenone (TNF); a pigment-dispersed type photoconductor comprising a binder agent and a pigment such as phthalocyanine; and a function-separating photoconductor comprising a charge generation material and a charge transport material in combination. Of these conventional organic photoconductors, special attention is paid to the function-separating photoconductor.
The mechanism for formation of latent electrostatic images on the function-separating photoconductor is as follows. When the photoconductor is uniformly charged, and then exposed to light images, the light passes through a transparent charge transport layer and is absorbed by a charge generation material in a charge generation layer. The charge generation material generates charge carriers after light absorption. The charge carriers thus generated are injected into the charge transport layer and are transported in the charge transport layer in accordance with an electric field which is generated by charging. When the charges on the surface of the photoconductor are neutralized, latent electrostatic images are formed on the surface of the photoconductor.
In the function-separating photoconductor, it is conventionally known and considered to be effective to use in combination (a) a charge transport material which exhibits main light absorption in the ultraviolet region and (b) a charge generation material which exhibits main light absorption in the visible region. Such a function-separating photoconductor is widely used in various image forming apparatus.
Most of the charge transport materials that have been developed are low-molecular weight compounds. Such low-molecular weight compounds, however, do not exhibit film-forming properties when used alone, so that the low-molecular weight compounds are usually used in the form of a dispersion or a mixture with an inert polymer to prepare a charge transport layer. The thus prepared charge transport layer comprising the low-molecular weight charge transport material and the inert polymer is lacking in mechanical durability, and therefore has a shortcoming that the charge transport layer tends to be abraded in the course of repeated use in an electrophotographic process. The above-mentioned shortcoming of the charge transport layer becomes a serious problem because there is a keen demand for the improvement of the durability of electrophotographic engine. The demand for a function-separating photoconductor with high mechanical durability is remarkably increasing.
One of the key factors that impair the life of the organic photoconductor for use with the electrophotographic process is mechanical abrasion of the photoconductive layer.
To be more specific, when the photoconductive layer of the photoconductor is abraded, the electric characteristics of the photoconductor, such as charging performance and light decay performance, are changed, and consequently a predetermined image forming process cannot be carried out. The result is that the desired image quality in hard copies cannot be maintained.
The abrasion of the photoconductive layer may occur wherever the photoconductor comes in contact with other image formation units disposed around the photoconductor. However, a particular attention must be paid to a cleaning unit, such as a cleaning blade or cleaning brush, for physically removing remaining toner from the surface of the photoconductor. Other units may abrade the surface of the photoconductive layer, but will not have any substantial effect on the actual life of the photoconductor.
The method for maintaining a low coefficient of friction on the surface of the photoconductor is roughly divided into two groups. One is to internally add a lubricating material to the photoconductive layer, or a protective layer when provided the photoconductive layer; and the other is to externally supply the lubricating material to the surface of the photoconductor so that the lubricating material is retained thereon. The latter proved to be more advantageous than the former in order to maintain high mechanical durability for an extended period of time. For example, according to Japanese Laid-Open Patent Application 6-83097, the surface top layer of the photoconductor contains a fluorine-containing resin. In this case, the atomic ratio of carbon atom to fluorine atom in the surface top layer is specified through the analysis of X-ray photoelectron spectroscope (XPS), or the analysis of the contact angle of pure water with the surface of the photoconductor. In this application, the lubricating material is internally added to the surface top layer of the photoconductor. This method can produce an effect of maintaining the performance of the photoconductor for a short period of time, more specifically, while making of several ten thousands of copies. However, the above-mentioned photoconductor cannot stand a long-term operation of making of several hundred thousands of copies. Namely, it is impossible to continue the copying operation until the limit of durability of the image forming apparatus itself.
On the other hand, the method of continuously supplying a lubricating material to the surface of the photoconductor is also conventionally known. For instance, there is provided a lubricating and abrasion member for eliminating the contaminant from the surface of the photoconductor and keeping the good condition of the surface of the photoconductor, as disclosed in Japanese Laid-Open Patent Application 56-51767. According to this application, abrasion of the photoconductor and supply of the lubricating material thereto are alternately carried out since a lubricating material and an abrasive material are alternately formed on the member.
In Japanese Laid-Open Patent Application 56-113183, a lubricant supplying member in a rotatable cylindrical form, which retains a lubricating material thereon is brought into contact with the surface of the photoconductor to supply the surface of the photoconductor with the lubricating material.
It is proposed in Japanese Laid-Open Patent Application 58-115468 that a step of coating a nonvolatile liquid such as a silicone oil on the surface of the photoconductor be provided in contact with the photoconductor prior or posterior to the step of cleaning the photoconductor using a cleaning blade that also comes in contact therewith. For example, a silicone rubber roller or blade impregnated with a silicone oil is used as a member for carrying out the above-mentioned coating step.
Japanese Laid-Open Patent Application 6-342236 describes that a lubricating material such as zinc stearate is supplied to the surface of an image bearing member such as a photoconductor via a charging roller.
In Japanese Laid-Open Patent Application 8-202226, it is proposed to supply a lubricating material such as zinc stearate to the surface of an image bearing member such as a photoconductor via a brush as controlling the coating amount of lubricating material.
It has thus become possible to maintain a low friction coefficient of the surface of the photoconductor for an extended period of time. However, when the coefficient of surface friction is excessively decreased in order to reduce the abrasion of the photoconductor, some side effects are produced. To be more specific, it is confirmed that the developing performance on the surface of the photoconductor is lowered so that the image density of the obtained image becomes insufficient, and abnormal images such as image blurring occur.
Accordingly, it is a first object of the present invention to provide an electrophotographic photoconductor which can exhibit excellent mechanical durability, and produce high quality images stably.
A second object of the present invention is to provide an image formation method employing the above-mentioned electrophotographic photoconductor.
A third object of the present invention is to provide an image forming apparatus employing the above-mentioned electrophotographic photoconductor.
A fourth object of the present invention is to provide a method for applying a lubricating material to the surface of a drum-shaped photoconductor so as to eliminate the conventional shortcomings.
The above-mentioned first object of the present invention can be achieved by an electrophotographic drum-shaped photoconductor comprising an electroconductive support and a photoconductive layer formed thereon, with a lubricating material being applied to the surface thereof in the shape of a plurality of bands spaced therebetween, constituting band-shaped lubricating material applied areas, extending along a circumference of the drum-shaped photoconductor in the direction perpendicular or substantially perpendicular to an axis of rotation of the drum-shaped photoconductor.
It is preferable that the band-shaped lubricating material applied areas satisfy such a dimensional and positional relationship of B/10 less than (A1+A2) less than 2B wherein A1 is the width of one of two adjacent band-shaped lubricating material applied areas, A2 is the width of the other band-shaped lubricating material applied area, and B is the distance between the center lines of the two adjacent band-shaped lubricating material applied areas in the direction of the circumference of the drum-shaped photoconductor.
The second object of the present invention can be achieved by an image formation method using an electrophotographic drum-shaped photoconductor comprising an electroconductive support and a photoconductive layer formed thereon, with a lubricating material being applied to the surface thereof in the shape of a plurality of bands spaced therebetween, constituting band-shaped lubricating material applied areas, extending along a circumference of the drum-shaped photoconductor in the direction perpendicular or substantially perpendicular to an axis of rotation of the drum-shaped photoconductor, the image formation method comprising the steps of uniformly charging the drum-shaped photoconductor, exposing the charged photoconducter to light images to form latent electrostatic images thereon, developing the latent electrostatic images to visible toner images using a developer, transferring the toner images to a transfer member, fixing the toner images onto the transfer sheet, and cleaning the surface of the drum-shaped photoconductor.
The third object of the present invention can be achieved by an image forming apparatus comprising an electrophotographic drum-shaped photoconductor which comprises an electroconductive support and a photoconductive layer formed thereon, with a lubricating material being applied to the surface thereof in the shape of a plurality of bands spaced therebetween, constituting band-shaped lubricating material applied areas, extending along a circumference of the drum-shaped photoconductor in the direction perpendicular or substantially perpendicular to an axis of rotation of the drum-shaped photoconductor; means for uniformly charging the drum-shaped photoconductor; means for exposing the charged photoconductor to light images to form latent electrostatic images thereon; means for developing the latent electrostatic images to visible toner images using a developer; means for transferring the toner images to a transfer member; means for fixing the toner images onto the transfer sheet; and means for cleaning the surface of the drum-shaped photoconductor.
The fourth object of the present invention can be achieved by a method for applying a lubricating material to the surface of an electrophotographic drum-shaped photoconductor for use in an electrophotographic image forming apparatus capable of producing toner images using a developer so that the lubricating material is applied to the surface thereof in the shape of a plurality of bands spaced therebetween, constituting band-shaped lubricating material applied areas, extending along a circumference of the drum-shaped photoconductor in the direction perpendicular or substantially perpendicular to an axis of rotation of the drum-shaped photoconductor.